The antibiotic rifamycin S is a derivative of the fermentation product rifamycin B. Rifamycin B is first oxidized to the intermediate compound rifamycin O. Rifamycin O then may be hydrolyzed to give Rifamycin S. This second step is generally carried out using aqueous sulfuric acid as a catalyst. This procedure for converting rifamycin O to rifamycin S is not satisfactory for a continuous process. Therefore, an improved process for carrying out this conversion is commercially desirable. The structure of rifamycin O and rifamycin S may be represented as follows: ##STR1##